Residents of New England may understandably look back at 2015 as the year of their never-ending winter. For the planet as a whole, though, this year could stand out most for putting to rest the “hiatus”— the 15-year slowdown in atmospheric warming that gained intense scrutiny by pundits, scientists, and the public. While interesting in its own right, the hiatus garnered far more attention than it deserved as a purported sign that future global warming would be much less than expected. The slowdown was preceded by almost 20 years of dramatic global temperature rise, and with 2014 having set a new global record high, there are signs that another decade-plus period of intensified atmospheric warming may be at our doorstep.
The most compelling argument for a renewed surge in global air temperature is rooted in the Pacific Decadal Oscillation
(PDO). This index tracks the fingerprint of sea surface temperature (SST) across the Pacific north of 20°N. A closely related index, the Interdecadal Pacific Oscillation (IPO), covers a larger swath of the entire Pacific. Both the PDO and IPO capture back-and-forth swings in the geography of Pacific SSTs that affect the exchange of heat between ocean and atmosphere (see Figure 1). We’ll use PDO as shorthand for both indexes in the following discussion.
The PDO typically leans toward a positive or negative state for more than a decade at a time. The positive phase, which features warmer-than-average SSTs along the U.S. West Coast, was dominant from the mid-1970s to the late 1990s. The PDO then flipped to a negative phase between about 1999 and 2013, with cooler-than-average SSTs along the West Coast. Figure 2 shows that even when a particular mode is favored, the PDO can still flip back to its opposite mode for periods of a few months or so.Figure 1.
Departures from average sea-surface temperature (degrees C) and wind (arrows) that typically prevail when the Pacific Decadal Oscillation is in its positive mode (left) and negative mode (right). Image credit: University of Washington
It’s not clear exactly what drives the PDO, but in some ways it can be viewed as a geographically expanded version of the SST patterns created by El Niño and La Niña, averaged over a longer time period. (See Figure 2.) It’s well-established that El Niño can raise global temperature for a few months by several tenths of a degree Celsius, as warm water spreads over the eastern tropical Pacific and mixes with the overlying atmosphere. Likewise, La Niña can act to pull down global average temperature, as cooler-than-average water extends further west than usual across the tropical Pacific. The PDO mirrors these trends, but over longer periods. When the PDO is positive, there are more El Niño and fewer La Niña events, and heat stored in the ocean tends to be spread across a larger surface area, allowing it to enter the atmosphere more easily. When the PDO is negative, SSTs are below average across a larger area, and global air temperatures tend to be lower. Figure 2.
Typical warm and cool anomalies in sea-surface temperature during positive PDO years (left) and El Niño years (right). The patterns are similar, though with differences in intensity over some regions. The anomalies are reversed for negative PDO and La Niña years. Image credit: University of Washington Climate Impacts Group.
Figure 3 shows a striking connection between favored PDO modes (top) and global air temperature anomalies (bottom). The vast majority of atmospheric warming over the last century occurred during positive PDO phases, with negative PDOs tending to result in flat temperature trends. It’s easy to see how an atmospheric warming “hiatus” could occur during a negative PDO phase.Figure 3.
PDO values (top) and global air temperature anomalies (bottom). Gray shading indicates positive PDO periods, when atmospheric warming was most evident. The NOAA PDO values shown here vary slightly from those discussed in the article, which are calculated by the University of Washington. Image credit: Jerimiah Brown, Weather Underground. Data sources:NOAA (top) and NOAA/NCDC
(bottom).From the AMS meeting
The hiatus was discussed at length in a series of talks
during the annual meeting of the American Meteorological Society last month in Phoenix. Jerry Meehl, from the National Center for Atmospheric Research (my former employer), gave a whirlwind 15-minute overview of hiatus-oriented research conducted over the last six years. Meehl’s talk can be viewed online
. More than 20 papers have studied the hiatus and its links to the PDO/IPO, according to Matthew England (University of New South Wales). Most of the flattening of global temperature during the hiatus can be traced to cooler-than-average conditions over the eastern tropical Pacific, which pulled down global averages. An emerging theme is that natural, or internal, variability in the tropical Pacific can explain at least half of the hiatus. NCAR’s Clara Deser presented new modeling evidence along these lines (see video online
). Other factors may be involved as well, including a series of weak volcanic eruptions that could explain a small part of the hiatus, according to a recent analysis
by Ben Santer (Lawrence Livermore National Laboratory).
One crucial point is that global warming didn’t “stop” during the hiatus: the world’s oceans actually gained heat at an accelerated pace. Trade winds blew more strongly from east to west across the Pacific, consistent with the tendency toward La Niña conditions, as described in this open-access article
by NCAR’s Kevin Trenberth and John Fasullo. Over parts of the central tropical Pacific, trade winds averaged about 3 mph stronger during 1999-2012 compared to 1976-1988. These speeds are higher than for any previous hiatus on record, bolstering the idea that other factors may have joined this negative PDO/IPO phase. The faster trade winds encouraged upwelling of cooler water to the east and helped deepen and strengthen the warm pool to the west—enough, in fact, to raise sea level around the Philippines by as much as 8 inches. Other parts of the deep ocean warmed as well. A new study
led by Dean Roemmich (Scripps Institution of Oceanography) maps the areas of greatest ocean heating from 2006 to 2013 and finds that significant warming extended to depths of greater than 6600 feet. What next for the PDO?
The PDO index
, as calculated at the University of Washington, scored positive values during every month in 2014, the first such streak since 2003. By December it reached +2.51, the largest positive value for any December in records that go back to 1900. The January value from UW was 2.45, again a monthly record. (NOAA calculates its own PDO values through a closely related methodology.)
Because the PDO can flip modes for a year or more within its longer-term cycle, we don’t yet know whether a significant shift to a positive PDO phase has begun. If trade winds weaken throughout this year, and positive PDO values persist, that’ll be strong evidence that a new cycle is indeed under way. The last time we saw a two-year streak of positive values was in 1992-93. If this occurs, and assuming no spikes in major volcanic activity, we could expect greater rises in global temperature over the next 10 to 15 years than we’ve seen during the hiatus. In addition, we should watch for El Niño to make its presence known more often.“I am inclined to think the hiatus is over, mainly based on the PDO index change,”
NCAR’s Kevin Trenberth told me. While Matthew England isn’t ready to offer such a prediction, he emphasized that any post-hiatus global temperature rise is likely to be fairly rapid
. Trenberth also commented on an interesting NOAA analysis (see Figure 4): “If one takes the global mean temperature from 1970 on, everything fits a linear trend quite well except 1998.”Figure 4.
When looking at global temperature over a full PDO cycle (1970s to 2010s), the overall rise becomes evident, despite the flattening observed in the last 15 years. Image credit: NOAA.
A record-strong El Niño occurred in 1998, providing an unusually powerful boost to global temperature and fueling years of subsequent declarations that “global warming stopped in 1998.” The record warmth of 2014 made it clear that global warming has no intention of stopping, and the next few years are likely to reinforce that point. Nevertheless, snowbound New Englanders, and millions of other easterners now dealing with record cold for so late in the year, may be wondering why eastern North America has seen so much cold and snow in the past few winters--especially this one--and how long that climatic quirk might continue. Stay tuned for a separate post on that topic.